Question

I’m trying to call an overloaded function which operates on function pointers that have parameters with default values.

void originalFunction1 (int a = 0) {printf("I'm #1 and a is %d",a);}
void originalFunction2 () {printf("I'm #2");}

void overloadedFunction (void (*fptr)(void))
{
  fptr();
}

void overloadedFunction (void (*fptr)(int))
{
  overloadedFunction( (void(*)(void)) fptr);
}

int main() 
{
  overloadedFunction(originalFunction1);
  overloadedFunction(originalFunction2);

  // output is:
  // I'm #1 an a is -1073743272
  // I'm #2
}

As the answers to this question points out, default values are not part of the function signature and also can’t be repeated during the (function pointer -) parameter definition.
As my example shows, they can be cast away for calling, but they will then not be initialized to their default value.

Is there any way to work around this?

I can’t modify the original function but I know the default value.
I can modify both the main overloaded function as well as the redirects.
The fptr’s will always only be called without parameters.
In reality, there are more overloaded functions as also the return type differs, but I can cast that away more easily.

Answer 1

The short answer is no. The cast is legal, but the only thing
you can do with the results is to cast them back to the orginal
type. Default arguments do not change the signature of the
function. They only enter into account at the call site, where
they are passed if the client doesn’t provide an argument. The
function itself is always called with the full set of arguments,
which it clearly won’t be if you call it through the results of
the cast.

The usual answer here, if you have control of the sources, is to
use overloading instead of default arguments. (I have heard
arguments that you should always use overloading instead of
default arguments.) So

void originalFunction1( int a ) { ... }
void originalFunction1() { originalFunction1( 0 ); }

instead of what you have.

Alternatively, you might be able to play games with templates:

template <typename FunctionPtr>
void overloadedFunction( FunctionPtr fptr )
{
    fptr();
}

I’d prefer the first solution if you can use it, however.